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Eighth International Water Technology Conference, IWTC8 2004, Alexandria, Egypt

197

MICRO CLIMATIC ENVIRONMENTAL CONDITIONS INSIDE GREENHOUSES WITH BUILT-IN SOLAR DISTILLATION

SYSTEM

Hassan E. S. Fath* and Khaled Y. Zakaria** Mechanical Eng. Dept., Faculty of Eng., Alex University, Alex., Egypt

(*Corresponding Author, e-mail: h_elbanna_f @ hotmail.com,

** e-mail: [email protected])

Abstract A numerical study has been carried out to investigate the micro climatic environmental conditions inside a greenhouse-distillation system, self sufficient of irrigating water. The greenhouse consists of the planting cavity, circulating air channels and roof solar distiller for the production of the rather modest rate of irrigating water. A turbulent - steady state flow, energy and humidity concentration equations have been solved using the numerical code FLUENT 6.1. Velocity vectors, steam function, isotherms and temperature and humidity distribution inside the greenhouse present the resultant micro climatic environmental conditions. The results have been presented for hot days, where cold and humid air (from evaporative cooler) enters the greenhouse from one side and circulated through the partially porous cavity (representing the plants) and flows through air flow channels and leaves from a vertical thermal chimney. The results show that, with the selected inlet flow conditions, the flow velocity, temperature, and relative humidity can be within comfort values for plants growth. The effect of some important environmental, design, and operational parameters on greenhouse microclimatic conditions has also been highlighted. Key Words: Agriculture, Greenhouse, CFD, HVAC

Eighth International Water Technology Conference, IWTC8 2004, Alexandria, Egypt 198

Nomenclature Cp Specific heat at constant pressure

C Constant

Dh2o Diffusion coefficient of water vapor into air

fj External body force in j direction

g Gravitational Acceleration (m/s2)

hj Sensible enthalpy

Jj Diffusion flux of species j

k Turbulent kinetic energy

p Static Pressure ( Pa )

S Modulus of the mean rate-of-strain tensor

Sm Mass source term

Sh2o Water vapor added to or removed from the air

Sct Turbulent Schmidt number

T Temperature ( K )

u x-direction velocity (m/s)

x Horizontal coordinate

y Vertical coordinate

Y Mass fraction of species

Greek Letters β Thermal expansion coefficient ρ Density (kg/m3)

µ Dynamic Viscosity µt Turbulent Viscosity

ε Turbulent dissipation rate ν Kinematics viscosity

λ Thermal conductivity γ Porosity

α Permeability

Subscripts i , j Index refer to the coordination directions i' , j' Index refer to different species

m Mass ref Reference

k Turbulence kinetic energy b Buoyancy

eff Effective


199

1- INTRODUCTION The design of integrated greenhouse-solar distillation system, presents an existing

possibility for support of small-scale agriculture activity where only sea or brackish water

are available. Solar distillation (stills) may in most cases be able to provide the rather

modest demand of greenhouse irrigating fresh water, consistent with clever choice of

crops, suitable thermal environment provided by the greenhouse and the efficient water

conservation technology. The part of solar radiation, essential for photo synthetic growth

of plants could be provided by having the greenhouse-distillation system walls made of

glass or other transparent material.

Several greenhouses incorporated with solar stills have been constructed and tested. For

example, two small greenhouses with solar stills were constructed in Texas in 1978; see

Malik et al. (1982). More recently, Chaibi (2000) in Tunisia, Srivastava et al. (2000) and

Ghosal et Al. (2002) in India presented an analysis of the solar still integrated in

greenhouse roof. Fath (1993, 1994, 1997) developed an integrated greenhouse; self

sufficient of energy and irrigation water. The system developed by Fath (1994), utilizes

the abundant solar energy (above that needed for photo synthetic growth of plants) to

produce the required irrigating fresh water by distillation system (stills). Locating the

stills at greenhouse roof, the greenhouse heat transfer load is reduced and so is the

ventilation rate and cooling requirements.

The control of agriculture greenhouses internal environment during both hot and cold

days is necessary for high plants quality & quantity production. The greenhouse inside air

conditions depends on; (i) incident solar radiation entering the greenhouse, (ii) inlet

ventilation air rate and conditions (temperature & relative humidity), (iii) the relative area

of evaporation (plants zone) that generates heat and water vapor, and (iv) the non

evaporative area (dry soil, paths, etc.) that may absorb the radiation and water vapor.

Ventilation inside greenhouses is an important factor of greenhouse environment control,

since it directly affects the transport of fresh air necessary for plant breathing and CO2

concentration needed for photo synthetic process. Although it varies from plants group to

another, typical plants basic comfort values are shown in Table (1), see Aldrich and

Bartok (1990), and UOC Report (1998). Plants produce water vapor by transpiration.


Low greenhouse humidity results in high plant transpiration rate which creates water

stress in plants. On the other hand, high humidity (above 95 %) activates the biological

elements that cause plants diseases. The ventilation process must, therefore, provide a

sufficient fresh air exchange necessary for plants growth and carry the heating/cooling

load. In addition, ventilation system should provide good mixing between the incoming

air and greenhouse air, and create an internal air movement that induces good heat and

mass transfer between the plants and air. The detailed study of the greenhouse micro

climatic conditions are, therefore, necessary for providing suitable conditions for plants

growth.

Table (1) Basic Comfort Values for Greenhouse Planting

Parameter Value

Temperature ( C ) 10 – 30

Relative Humidity ( % ) 25 – 80

Air Velocity with plant (m/s) 0.1 – 0.25 (0.5 maximum)

The implementation of Computational Fluid Dynamics (CFD) codes offers a powerful

design and analysis tool for studying the internal flow, temperature and humidity

distribution where fluid flow and heat and mass transfer play an important role. With

CFD codes, the significance and effect of various environmental, design, and operational

parameters can be analyzed so that better micro climatic conditions of plants zone can be

achieved. CFD is currently gaining popularity as an ideal tool for obtaining a qualitative

and quantitative assessment of the performance of greenhouses facilities leading to the

optimization of new designs and overcoming the drawbacks of existing designs. For

example, Boulard et al. (1999) used the commercial CFD-2000 package to study the air

flow and temperature patterns induced by buoyancy forces through greenhouse openings.

The researches observed that the incoming air stream through the lower section of the

opening swept over the greenhouse walls in a large convection loop before escaping

through the upper section of the opening. These results were used as a guide to improve

greenhouse climatic control and vent positioning and design. The authors indicated that


201

by comparing with experimental visualization, CFD results were only a first

approximation of the air flows, because the plants further complicates the internal

convection in practice. To get more realistic description, the authors suggested simulating

the aerodynamic situation in the presence of plants as porous medium. In their review

paper, Boulard et al. (2002) presented a complete panorama of the studies pertaining to

air movement inside the various greenhouse types. The authors focused on recent studies

dealing with plant-air interaction, particularly the leaf boundary layer climatic and the air

flows within the crop canopy. Despite the difficulties of modeling, the authors show that

simulations involving CFD software are becoming more realistic and able to describe

with good accuracy the main features of the distribution climatic inside greenhouse.

Bartzanas et al. (2002) presented a numerical simulation of the airflow and temperature

distribution in a tunnel greenhouse. The authors used also the commercial CFD 2000

package to analyze the influence of an insect-proof screen on air and temperature pattern

inside the greenhouse. The presence of the screen was found to strongly reduce air

velocity inside the greenhouse resulting in a significant temperature rise.

Kacira et al. (1998) used the Computational Fluid Dynamic software FLUENT to predict

the two-dimensional air flow and temperature pattern in a multi-span saw-tooth

greenhouse for various roof and side vents. Reichrath and Davies (2001) simulated a two-

dimensional full-size commercial multi-span greenhouse with the help of FLUENT-

5.3.18, to predict the pressure distribution on the external side of the glasshouse roof. The

numerical results were successfully validated by experimental work conducted on a 52

span Venlo-type glasshouse. The agreement between numerical and experimental results

gave the confidence for glasshouse simulation. Howell and Potts (2002) used FLUENT

version 5.4 package to simulate a test room with extensive experimental measurements to

verify the code. The radiation absorption characteristics of air, due to the content of water

vapor in the atmosphere, were incorporated into the simulation for more realistic

prediction. Lee and Short (2000) studied the dynamic influence of tall crops on airflow.

The presence of plants was studied for their influence on natural ventilation rate and air

flow distribution. The porous medium approach was used to model the dynamic effect of

the crop on the flow.


The motivation for the present work arises from the fact that an integrated greenhouse-

distillation system is to be constructed as a self sufficient agriculture system of irrigating

water. The system is part of a research project, partially funded by EC (2003), to be

implemented in remote area, where the fresh water shortage represents a problem to the

local society. In an attempt to predict the micro climatic environment inside the

greenhouse-still system, the CFD package, FLUENT-6.1 will be used as the design tool.

This paper presents the code development and the pre construction steady state, turbulent

flow case in hot climates and high solar intensity days.

2. PRESENT SYSTEM DESCRIPTION

2.1 System Configuration

The two main objectives of the proposed system are: (1) to utilize the excess solar energy

(above that required for the plants photo synthetic process) to produce the rather modest

demand of the system irrigating fresh water, and (2) to create a suitable micro climatic

conditions for plants growth. This is carried out by installing a solar distillation unit

(solar still) situated in the GH roof. The still utilizes the part of solar energy absorbed by

saline water in basin for water production and, in addition, reduces the heat load on plants

cavity in hot-sunny days. Figure (1) shows the 2-D schematic diagram of the greenhouse-

still integrated system and its conceptual configuration. The greenhouse (GH) cross

section has a rectangular plants growth cavity with triangular roof, Figure (1). The main

components of the system, as shown in the figure, are; a- 3.0 m x 5.0 m plants cavity with

1.5 m x 3.0 m plants growth zone, b- Solar still(s) of 5.5 m wide semi transparent basin,

with 30o inclined transparent still cover, c- 1.0 m wide air flow vertical channels,

followed by 0.5 m wide horizontal channel and ending with 0.5 m wide thermal chimney.

The greenhouse (GH) is to be built of a transparent material (of maximum transmisivity

to maximize the amount of transmitted solar energy).

2.2 Hot Days Operation

In hot days, ambient air enters the greenhouse cavity through evaporative cooler pads,

(5), for ventilation and air cooling requirements. The inlet air is partially cooled (and


203

humidified) and, therefore, cools the air inside the GH plants cavity. The air gains some

of the cavity heat (inlet solar energy, plants heat and other internal energies) and moisture

(from plants transpiration, (6) and irrigation system, (17), within the cavity), and leaves

the cavity to the vertical channel, (9), then flows through the horizontal channel, (8).

Based of the ventilation air requirements for plants, air is vented to the atmosphere

through a thermal chimney, (4), where the chimney damper is opened and the cavity

damper is closed. In the still, natural air circulation takes place where heated air carries

the hot vapor evaporated from the still basins, (7) to the colder triangle cover, (3). The

vapor is condensed at the still cover to form the distillate (product water), where it is

collected in the product water collection trays (troughs), (10).

Figure 1-a Greenhouse with Built-in Roof Still (Conceptual Design)


Figure (1-b) Greenhouse with Built-in Roof Still

(Configuration and Boundary regions)

3- GOVERNING EQUATIONS The basic assumptions for the CFD simulation include two dimensional, fully turbulent, non-isothermal, incompressible flow. The canopy region will be treated as a homogeneous porous medium. The flow inside this porous medium will also be assumed to be fully turbulent. The industry standard for turbulence simulation is the k-ε model because of its robustness at a relatively low computational cost. The transport equations for incompressible turbulent flow in tensor notation take the following form (Fluent User’s Guide):

3.1 Continuity Equation

mi

i Sxu =

∂∂ )(ρ

(1)

The mass source term mS is added to or removed from the continuous phase; due to evaporation or condensation of liquid droplets.

0.5 m

�� m

�� m

3.0 m

5.5 m

�� m

�� m

�.5 m

�� m

�� m

�� m

�� m

0.5 m

30°

0.5 m

�� m

A B B

D

E

C

F

G

H

I

K

Crop

J

L

B B

�� m


205

3.2 Momentum Equation The Reynolds-averaged Navier-Stokes (RANS) equations governing the flow take the form:

jj

ji

i

j

j

i

jij

ji fx

uu

x

u

xu

xxp

x

uu+

∂−∂

+��

�

��

�

��

�

�

�

∂∂

+∂∂

∂∂+

∂∂−=

∂∂ )()( ''ρ

µρ

(2)

The Reynolds stresses ''ji uuρ are related to the mean velocity gradients by employing the

Boussinesq hypothesis:

��

�

�

�

∂∂

+∂∂=

i

j

j

itji x

u

xu

uu µρ '' (3)

where µt is the so-called turbulent viscosity defined as:

ερµ µ

2kCt = (4)

with

''

21

ji uuk = (5)

and

i

j

j

i

x

u

xu

∂∂

∂∂

=''

νε (6)

The Boussinesq model is employed for the calculation of the buoyancy force, fj where the fluid model density is not dependent on temperature, pressure or additional variables. The local density variation is defined as:

( )[ ]refTT −−= βρρ 1' (7)

Calculation of the turbulent kinetic energy, ,k and dissipation rate, ,ε requires two additional partial differential equations which contain four unknown coefficients in addition to µC of Eq. (4). The two additional partial differential equations take the form:

ρεσµµρ −++

��

�

��

�

∂∂

��

�

�+

∂∂=

∂∂

bkjk

t

ji

j

GGxk

xuk

x)( (8)

( )k

CGCGk

Cxx

ux bk

j

t

ji

j

2

231)(ερεε

σµµερ εεε

ε

−+��

�

��

�

∂∂

��

�

�+

∂∂=

∂∂

(9)


In Eq. (7) and (8), the terms �� and �� are the generation of turbulent kinetic energy due to mean velocity gradient and due to buoyancy and could be described as follows:

i

jjik x

uuuG

∂∂

−= ''ρ (10)

and �� is evaluated in a manner consistent with the Boussinesq hypothesis: 2SG tk µ= (11)

where S is the modulus of the mean rate-of-strain tensor, defined as :

2 ij ijS S S= (12)

the buoyancy effect takes the form :

it

tik x

TgG

∂∂=

Prµβ (13)

where the coefficient of thermal expansion ,β is defined as :

pT ��

�

�

∂∂−= ρ

ρβ 1

(14)

The constants in the above equations take the following values:

C1ε = 1.44, C2ε = 1.92, Cµ = 0.09, σk = 1.0, σε = 1.3, Prt = 0.85

The degree to which ε is affected by the buoyancy is determined by the constant C3ε . This constant is calculated according to the following relation:

3 t anhv

Cue = (15)

Where v is the component of the flow velocity parallel to the gravitational vector and u is the component of the velocity perpendicular to the gravitational vector. In this way, C3ε will become 1 for buoyant shear layers for which the main flow direction is aligned with the direction of gravity. For buoyant shear layers that are perpendicular to the gravitational vector, C3ε will become zero.

3.3 Energy Equation For the temperature distribution calculations, the energy equation can be written as:

( )[ ] hj

jjjt

tp

ji

j

SJhxTc

xpEu

x+

��

�

��

�−

∂∂

��

�

�+

∂∂=+

∂∂

Pr

µλρ (16)


207

where λ is the thermal conductivity of the fluid, jJ is the diffusion flux of species j, and Sh is any volumetric energy source. The term E in the previous equation is defined as follows:

+=j

jj

uYhE

2

2

(17)

where sensible enthalpy h is defined for ideal gases as:

�=T

T jpjref

dTch , (18)

and �� is the mass fraction of species j .

3.4 The Species Transport Equation The species transport equation of water vapor into air is written in the form:

( ) ohj

oh

t

toh

jioh

j

Sx

YSc

Dx

uYx 2

222 +

��

�

��

�

∂∂

��

�

�+

∂∂=

∂∂ µρρ (19)

where, Sh2o is the water vapor added to or removed from the air due to evaporation or condensation. The constant Dh2o is the diffusion coefficient of water vapor into air which is equal to 2.88 x 10-5; Sct is the turbulent Schmidt number which is equal to 0.7.

3.5 Governing Equations for Porous Media When dealing with any porous medium, Fluent gives you the option to choose between solving the porous region with superficial velocity or a more accurate model called physical velocity. The relationship between these velocities could be described as:

physicalerficial uu γ=sup (20)

where � is the porosity of the media defined as the ratio of the volume occupied by the fluid to the total volume. The superficial velocity values within the porous region remain the same as those outside of the porous region. This limits the accuracy of the porous model where there should be an increase in velocity throughout the porous region. For more accurate simulations of porous media flows, it becomes necessary to solve for the true, or physical velocity throughout the flow field, rather than the superficial velocity. The following sets of equations represent the governing equations inside the porous region:


3.5.1 Continuity Equation

mi

i Sxu =

∂∂ )(γρ

(21)

3.5.2 Momentum Equation Porous media are modeled by the addition of a momentum source term to the standard fluid flow equations. The source term is composed of two parts: a viscous loss term

( iuma

), and an inertial loss term ( 2

12 mag iC u ur ). Where � is the permeability and �� is the

inertial resistance factor. The value of �� and �� should be chosen to represent the crop under consideration. The final form of the momentum equation will be:

j

ji

i

j

j

i

jij

ji

x

uu

x

u

xu

xxp

x

uu

∂−∂

+��

�

��

�

��

�

�

�

∂∂

+∂∂

∂∂+

∂∂−=

∂∂ )()( ''ρ

γµγγγρ

��

�

� +−+ imagii uuCug ραµργ

21

2 (22)

3.5.3 Energy Equation The thermal conductivity of the porous medium is calculated based on the following relationship

( ) sfeff λγγλλ −+= 1 (23) The final form of the energy equation will be:

( )[ ] hj

jjjt

tpeff

jffi

j

SJhxTc

xpEu

x+

��

�

��

�−

∂∂

��

�

�+

∂∂=+

∂∂

Pr

µλργ (24)

where:

fE is the total fluid energy.

3.6 Scalar Quantities The rest of the transport equations could be modeled as follows:

φγφγφγρ Sxx

ux jj

ij

+��

�

��

�

∂∂Γ

∂∂=

∂∂

)( (25)

where φ is the scalar property under consideration (k, �, or Yh2o.)


209

4 NUMERICAL SOLUTION

4.1 Solution Domain and Boundary conditions Figure (2) shows the solution domain that represents the Hot-Day configuration of the greenhouse. Table (2) summarizes all the boundary conditions used to simulate such greenhouse-still configuration. Temperature values of greenhouse walls have been selected after the midday results of Fath (1994). Other values of parameters as transpiration rate are obtained from Aldrich and Bartok (1990), and UOC Report (1998).

Figure 2 Schematic Diagram Representing the Greenhouse-Still with Grid Regions

4.2 Porous Medium The crop was simulated using the porous medium approach. The plants inside the greenhouse are assumed to be well-developed tomato crop. Hydrodynamic properties of that crop were studied by Haxaire (1999) where the same values were used to simulate the tomato crop. The permeability of the crop � was taken to be 0.395 and the coefficient C2 of equation (22) was taken to be 1.6 which represent a tomato crop with a drag coefficient CD = 0.02 and a Leaf Area Index ( LAI ) of 4.0. The solar energy absorbed by the crop was estimated to be 85 W/m3 (the estimated solar energy requirement for photo-

I

II III

V VI

IV


synthetic process). The distiller is assumed to absorb the rest of the incident solar energy on the greenhouse for irrigating water production. The crop transpiration was taken to be 0.0002 kg/m3 of the crop and was simulated as a uniform mass source inside the mass transport equation.

4.3 Distiller The evaporation and condensation of the water inside the distiller was simplified by a source and sink terms of water vapor. At the bottom of the distiller, a water vapor source term of 0.0001 kg/m2 s was added to a thin layer of fluid in the vicinity of the bottom wall of the distiller. At the still cover, a modulated sink term was added so that the evaporation mass rate will be equal to the condensation rate at the still cover.

4.4 Numerical Method The numerical solution of the transport equations were carried out with a user-enhanced, structured finite-volume based program Fluent 6.1 (Fluent Incorporated, Lebanon, NH 03766), with segregated solver, good robustness, and rapid convergence. Fluent 6.1 employs an unstructured control volume mesh with rectangular meshes on the surface of the geometry. The greenhouse was subdivided into 5 zones. Each zone was first assigned an unstructured grid with near-wall refinement techniques. The segregated solver was used until the solution converged. The solution grid was then refined based on the velocity gradient and the problem was then resolved. This process continued until the maximum velocity gradient inside the domain was less than 0.01. Figure 2 shows the solution grid for a total of 100,000 cells. Further refinements were also tested and found not to significantly change the simulation. The convergence criteria for acceptable level of error are the normalized residuals which provide a way of establishing how well conservation has been maintained. For this study, if the normalized residuals for all variables were below 1.0E-4 then solution was assumed to be converged. The solution was performed on a regular PC and 10 hours were enough to reach a converged solution.


211

Table 2 Boundary conditions at different greenhouse regions

Region Momentum T K ( °°°°C )

Yh2o

kg(w)/kg(air)

A No Slip Wall 308 (35)

02 =∂

∂x

Y oh

B No Slip Wall 313 (40) - 0.1 g/s.m2

C No Slip Wall 308 (35)

02 =∂

∂x

Y oh

D No Slip Wall 0=∂∂

yT

02 =∂

∂y

Y oh

E No Slip Wall 303 (30)

02 =∂

∂x

Y oh

F Inlet: �� m/s, �� m/s�� m2/s2�� m2/s3 298

(25) 0.015

G No Slip Wall 303 (30)

02 =∂

∂x

Y oh

H No Slip Wall 308 (35)

02 =∂

∂x

Y oh

I Out Flow 0=∂∂

yT

02 =∂

∂y

Y oh

J No Slip Wall 328 (55) + 0.1 g/s.m2

K No Slip Wall Conduction 0.008 m Glass Wall

02 =∂

∂y

Y oh

L No Slip Wall Conduction 0.008 m Glass Wall

02 =∂

∂x

Y oh


5- RESULTS AND DISCUSSION

The development of the micro climate environment inside the greenhouse - still integrated system, in hot days, is shown in Figures (3) to (11).

Figure (3) illustrates the convergence properties development. This includes the values of the continuity, momentum, temperatures, turbulent quantities and humidity inside both the greenhouse and its roof still. It is important to note that the domain of solution included both the GH and the distiller configurations, i.e. both domains were solved simultaneously. This actually represents how strong the code used is. The code was able to handle two different problems (free convection inside the still and mixed convection inside the greenhouse cavity and air flow channels) without any problem. Due to the large number of cells used (100,000) the convergence required about 6000 iterations to be reached. The jump in the convergence values at iteration 3600 was due to the activation of the porous medium representing the crop. This technique was found to accelerate the overall convergence of the problem. About 12 hours of computation time was required to achieve a converged solution on an 800 MHz PIII personal computer.

Figure (3) Convergence history of the GH-Distiller System

(both solved simultaneously)

Figures (4) and (5) show the flow pattern within the greenhouse and the roof still in a form of velocity vectors and stream functions respectively (colored by velocity magnitude). As expected, and due to the symmetric boundary conditions for the roof still, a symmetric flow pattern and a mirror image is obtained around the vertical still centerline. For the greenhouse, two vortices are developed near the entrance two corners of the plant cavity with another two vortices at entrance of each flow channel. Due to the


213

higher resistance of plant to flow, the recirculation vortex developed at the entrance bottom corner has lower velocity than that developed within the top corner vortex. The vortices are more apparent in the stream function contours shown in Figure (5). The existence of air re-circulating and secondary flows within the plants should not affect the quality of the internal conditions created. The detailed velocity distribution at different locations of the plant cavity, within and out side plants zone is shown in Figure (6). The Figure shows that within the plants zone, the air velocity varies between 0.1 - 0.2 m/s which perfectly suits the plants growth as indicated in the comfort values for the plant growth, see Table (1).

Figure (4) Velocity Vectors inside the GH-Distiller System (Coloured By Magnitude, m/s)


Figure (5) Contours of Stream Function inside the Greenhouse-Still System

0

0.5

1

1.5

2

2.5

3

0 0.2 0.4 0.6 0.8 1

Velocity m/s

Hig

ht m

Inlet0.00.51.01.52.02.53.0Outlet

Figure (6) Velocity Distribution inside GH Cavity


215

Figure (7) shows the temperatures contours within the greenhouse-still system. Due to the small walls temperature differences, the gravity appears to be not effective within the greenhouse plant cavity. This is mainly because the hottest wall is the top wall (due to solar energy absorption). Hot air layer is trapped near the roof giving no room for natural currents to develop within the plants cavity. The temperature distribution within the plants cavity, Figure (8), shows that temperature values slightly varies within the plants cavity and very well in-consistent within the plants growth requirement. On the other hand, the roof still flow pattern, as shown both by the flow stream function, velocity vectors, Figures (4 & 5) and temperature contours, Figure (7), show the free convection as the only driving force for natural flow circulation within the still. Higher air temperature rises upward from the still basin (heat & mass source) and near still center, and cold air flow down-ward along the glass cover (heat & mass sink). Two main large vortices, representing the natural air circulation inside the still, are developed as shown.

Figure (7) Temperature Contours inside the Greenhouse-Still System


0

0.5

1

1.5

2

2.5

3

297 298 299 300 301 302 303 304 305 306

Temperature K

Hig

ht m

Inlet0.00.51.01.52.02.53.0Outlet

Figure (8) Temperature Distribution inside the Greenhouse Cavity


217

Figures (9 & 10) show the contours and distribution of the air relative humidity. Contours of the mass fraction within the system are shown in Figure (11). The region of maximum humidity is near the right hand bottom corner (outlet flow) where vapor concentration is highest (due to additional vapor generated from plant transpiration). The relative humidity within the plant zone varies between 80 % to 83 % which lies within the acceptable growth comfort values.

Figure (9) Contours of Relative Humidity within the Greenhouse-Still System


0

0.5

1

1.5

2

2.5

3

0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9

Relative Humedity

Hig

ht m

Inlet0.00.51.01.52.02.53.0Outlet

Figure (10) Relative Humidity Distribution within the Greenhouse Cavity

Figure (11) shows the contours of the mass fraction inside the greenhouse cavity. The figure shows clearly the increase of water mass fraction inside the plants zone in the stream-wise direction due to the water vapor generation inside the plants zone.


219

Figure (11) Mass Fraction Distribution within the Greenhouse Cavity


Different environmental, design, and operational parameters can influence the microclimatic conditions of plants cavity and greenhouse effectiveness. The environmental parameters include the incident solar intensity, ambient temperature, relative humidity and wind speed. For the steady state study, these parameters can be combined to affect both greenhouse-still walls temperatures and inlet air conditions. The design parameters include the greenhouse-still dimensions and its material properties. Higher thermal chimney, for example, may enhance natural draft and reduce fan resistance. However, it will negatively affect the greenhouse structure integrity and cost. Similarly, the vertical and horizontal airflow channels will slightly affect the air flow resistance and fan power if the channels width changed. In addition, pressure drop across the evaporative cooler pads and across the plants over ride these channels pressure drop. Greenhouse-still material properties (transmisivity & absorbitivity) will affect walls temperatures and amount of heat transmitted to the (and then generated from) the plants. These amount of heat could however be controlled by adjusting the semi transparent shutters surrounding the plants cavity, see Figure (1.a). On the other hand, inlet and outlet air opening (to and from plants cavity) may significantly affect the microclimatic conditions. For the steady state study, these inlet and outlet air opening parameters will be considered. The operational parameters include evaporative cooler performance parameters; such as air flow rate (which could vary with time due to pads scale deposits) and cooling and humidification effectiveness. These parameters could ultimately be incorporated as inlet air velocity and conditions. Another important operation parameter is the plant configuration (plants growt zone dimension within the plant cavity & plants porosity), and plants transpiration rates. Plants area (width) is constraint by a maximum value of 50 % of cavity area, UOC (1998). Similarly, the plants porosity is limited to allow air flow through the plants leaves. However, plants height and transpiration rates may vary from plant type to another. From the above discussion, a summary of the main effective environmental, design and operational parameters, on greenhouse microclimatic conditions, is given in Table (3) with their extreme values. For such selected extreme case, the right hand wall is removed to facilitate air outlet flow from the plants cavity. Figures (12) to (15) shows the main thermo-fluid conditions of the greenhouse still system. The most significant difference from the above studied normal case is that a large re circulating vortex is developed in top of the plant zone as shown in the velocity vector and stream function diagrams, Figures (12) & (13). The relative humidity in this region, Figure (14), shows very high values (above 100% since no condensation was considered in the modelling), which will activate the biological elements that cause plants diseases. The temperature distribution within the plants zone, Figure (15), shows also a 3-4 C air temperature increase above inlet value in the plants top area.


221

Table (3) The main parameters consider to effective for the system performance.

Parameter Normal Value

Extreme Value

Remark

GH wall temperature (C) Table (2) + 10 C Maximum possible Inlet Air Conditions Velocity (m/s) Temp. (C ) Humidity Ratio (kgw/kga)

0.5 25 0.015

0.1 +10 0.03

Minimum allowable Maximum possible Maximum possible

GH Dimension Inlet Air Width (m) Outlet Air Width (m)

1 1

2 3

Maximum available Maximum available

Plants zone Conditions Transpiration Rate (g/m3) Height (m)

0.2 1.5

0.4 3.0

doubled Plants to the top

Figure (12) Velocity Vectors inside the GH (extreme case, Table (3))


Figure (13) Stream Function inside the GH (extreme case, Table (3))

Figure (14) Relative Humidity inside the GH (extreme case, Table (3))


223

0

0.5

1

1.5

2

2.5

3

306 308 310 312 314 316 318 320

Temperature K

Hig

ht m

Inlet0.00.51.01.52.02.53.0

Figure (15) Temperature Distribution within the Plants zone (extreme case, Table (3))

In order to avoid this unacceptable micro climatic condition, the transpiration rate of 0.2 g/m3 is to be a limiting value. Studying this condition, with low inlet velocity 0.1 m/s, the RH still shows high un acceptable values, Figure (16) specially in the top area of plants zone.


Figure (16) Relative Humidity (0.2 g/m3 Vapor Generation Rate, 10 °°°°C Higher Temperatures & 0.1 m/s inlet velocity)

The air velocity should, now, be increased back to 0.5 m/s to eliminate the top vortex and unacceptable micro climatic conditions. The results of these conditions are shown in Figures (17) to (19). Figures (17) & (18) show the velocity vectors and the stream function where the plants top vectors are eliminated. Figure (19) shows the RH within the plants is back to be below 90 %. These limiting conditions are given in Table (4).

Table (4) The limiting operating conditions.

Parameter Limiting Value GH wall temperature (C) + 10 C Inlet Air Conditions Velocity (m/s) Temp. (C ) Humidity Ratio (kgw/kga)

0.5 +10 0.03

GH Dimension Inlet Air Width (m) Outlet Air Width (m)

2 3

Plants zone Conditions Transpiration Rate (g/m3) Height (m)

0.2 3.0


225

Figure (17) Velocity Vectors inside the GH (Limiting extreme conditions)

Figure (18) Stream Function inside the GH (Limiting extreme conditions)


Figure (19) Relative Humidity inside the GH (Limiting extreme conditions)


227

6 CONCLUDING REMARKS 1. A numerical simulation of the steady state turbulent flow, temperature and

humidity distribution inside an agriculture greenhouse, with built-in solar distillation system, is presented. The micro climatic environmental conditions results have been presented for the pre-construction of a greenhouse-stills integration system. The results are presented for hot days where cold and humid air (from evaporative cooler) enters the greenhouse and leaves, through a partially porous cavity (representing the plants), to flow channels and thermal chimney.

2. Turbulent flow, energy and humidity concentration equations have been solved using the numerical code FLUENT 6.1. The results have been presented in the form of velocity vectors, stream function, isotherms and humidity and mass fraction contours. The developed computer package proved to be an effective tool for the study and analysis of the micro climatic conditions of a pre-constructed greenhouse-stills system for optimum plants growth.

3. With the selected inlet flow conditions, the flow velocity, temperature, and relative humidity were found to be within the comfort values for plants growth.

4. The effect of some important environmental, design, and operational parameters on greenhouse microclimatic conditions and economics have been analyzed. The greenhouse can accept harsher environmental conditions that make its wall temperatures and inlet air temperatures 10 C higher that given in Table (2). The inlet velocity should not decrease below 0.5 m/s to avoid re circulating vortices that increases the RH within the plants growth zone to un acceptable values.

5. The effect of some important environmental, design, and operational parameters on greenhouse microclimatic conditions has been analyzed. The greenhouse can accept harsher environmental conditions that make its wall temperatures and inlet air temperatures 10 C higher that given in Table (2). The inlet velocity should not decrease below 0.5 m/s, and the plants transpiration rate should not be higher than 0.2 g/m3 in order to avoid re circulating vortices that increases the RH within the plants growth zone to un acceptable values.


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Bartzanas T. (2002), Boulard T. and Kittas C. “Numerical Simulation of the airflow and Temperature Distribution in a Tunnel Greenhouse Equipped with Insect-Proof Screen in the Opening”, Computers and Electronics in Agriculture, Vol. 34, Issues 1-3, pp 207-221.

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Haxaire R., 1999. Caractérisation et modélisation des écoulements d’air dans une serre. Ph.D. Thesis, University of Nice Sophia Antipolis, France.

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Kacira M., Short T. H, Stowell R. R. (1998), “A CFD Evaluation of Naturally Ventilated, Multi-Span, Sawtooth Greenhouse”, Transaction of the ASME, Vol. 41, No. 3, pp 833-836.

Lee I. and Short T. H. (2000) “Two-dimensional Numerical Simulation of Natural Ventilation in a Multi-Span Greenhouse”, Transaction of the ASME, Vol. 43, No. 2, pp 745-753.

Malik M. A. S., Tiwari G. N., Kumar A. and Sodha M. S. (1982) “Solar Distillation”, Pergamon Press.

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